DC potential shift preceding repetitive perceptually relevant stimuli and its possible origin

The Necker cube pattern was flashed repetitively with regular, 5.12 s interstimulus intervals and EEG activity was recorded by means of non-polarizable Ag/AgCl electrodes and averaged. The subjects reported the actual subjective interpretation of the pattern by pushing one of two buttons, i.e. they were motivated to interpret each stimulus. A negative potential shift (lasting about 2.5 s increasing in a more or less linear way and, reaching the final amplitude of about 11 mV), developed gradually before the stimulus onset. Positive correlation was found between the duration and amplitude of DC shift. The phenomenon described might reflect cortical processes related to expectation of a regularly repeated perceptually relevant stimulus and may be considered a version of CNV. A hypothesis interpreting the DC shift as the consequence of neuroglial processes is presented.